Electronic and opto-electronic devices such as organic light-emitting diodes (OLEDs) are known in the art. Those OLEDs are also referred to as organic electroluminescent (EL) devices which generally comprise an organic electroluminescent material sandwiched between two electrodes. Generally, the organic electroluminescent material is a multilayer structure comprising an electron transport layer, an electroluminescent layer and a hole transport layer. Upon application of an electrical current, the material radiates light generated by recombination of electrons and holes in the organic material. However, the organic luminescent materials are sensitive to impurities, oxygen, and humidity. Further, in some electronic or opto-electronic devices, the electrodes influence the intensity, stability, and reliability of the device. Organic electroluminescent devices (materials and structure) are known in the art as, for example, disclosed in U.S. Pat. No. 4,356,429, U.S. Pat. No. 5,593,788, or U.S. Pat. No. 5,408,109, the disclosures of which are incorporated herein by reference for all purposes.
With multilayer device architectures now well understood and widely used, a remaining performance limitation of OLEDs is the electrode. The main figure of merit for electrode materials is the position of the electrode Fermi energy relative to the relevant organic molecular energy levels. In some applications it is also desirable for an electrode to assist light extraction. Electrodes should also be chemically inert with respect to the adjacent organic material to provide long term stability of the electroluminescent device.
Much attention has been paid to the cathode, largely because good electron injectors are low work function metals which are also chemically reactive and oxidize quickly in atmosphere, limiting the OLED reliability and lifetime. Much less attention has been paid to the optimization of the anode contact, since conventional ITO anodes generally outperform the cathode contact leading to an excess of holes. Due to this excess, and the convenience associated with the conductivity and transparency of indium-tin-oxide (ITO), improved anodes have not been as actively sought as improved cathodes.
Problems concerning sufficient hole injection and operational stability arose with the use of organic electroluminescent devices. Some problems have been relieved by a fluorocarbon treatment of the device's anode. U.S. Pat. No. 6,127,004 relates to a method of forming an electroluminescent device comprising the steps of providing a substrate having a top surface coating with a material including an anode having indium-tin-oxide (ITO); and forming an amorphous conductive layer over the anode by providing a fluorocarbon gas in a radical source cavity and subjecting such fluorocarbon gas to a reduced pressure in a range of 0.1 to 20 mT. Further an RF field is applied across the fluorocarbon gas in the radical source cavity to form a plasma having CFx radicals and the CFx radicals are deposited onto the anode forming an amorphous CFx conductive polymer layer on the anode. Then a plurality of layers over the amorphous CFx conductive polymer layer with such layers including at least one organic electroluminescent layer and a cathode over the electroluminescent layer are formed.
U.S. Pat. No. 6,208,075 relates also to an organic electroluminescent device which has a conductive fluorocarbon polymer layer disposed over an anode and U.S. Pat. No. 6,208,077 shows a thin non-conductive fluorocarbon polymer layer disposed over the anode. The mentioned fluorocarbon polymer layers are applied because of their transport properties and serve therefore as hole injection layers. The fluorocarbon polymer layers adheres preferably on anodes containing oxygen, e.g. ITO, otherwise using other materials lead to unstable device performance.
The international application with international publication number WO 99/39393, presently assigned to the assignee of the instant application, relates to an organic light emitting device having in order an anode, a barrier layer, an anode modification layer, an organic region, and a cathode. The anode modification layer is in direct contact with the organic region. The barrier layer is arranged to separate the anode modification layer form the anode but however this layer interferes the injection as it shows barrier properties.
An opto-electronic device can work either as a top emission device or a bottom emission device, also referred to as back emission device. For bottom emission devices, the anode should be nearly transparent such that the emitted light can pass through the anode.
Indium-tin-oxide (ITO) has been widely applied as anode because it forms nearly transparent layers. ITO has the disadvantage that it can partly react with the layer on top, e.g. the hole transporting organic material. This can lead to a shortening of the lifetime of the device. To circumvent the shortening, usually a buffer layer, e.g. CuPc, is used between the anode and the organic material, but on the other hand the buffer layer has a high resistance and interferes the injection. The use of a fluorocarbon polymer layer, as mentioned above, allows to discard the buffer layer.
For top emission devices molybdenum or platinum have been applied. These materials are not transparent and have a strong optical absorption. The reflection index of platinum is not optimal. Silver (Ag) and aluminum (Al) have a high reflectivity but a lower work function and are therefore unsuitable as anode materials. In general, materials having a high work function are rendered to be best suited as anode material. Low work function materials, e.g. Al, generally are highly chemical active even when covered with a buffer layer, e.g. CuPc, and consequently those materials are therefore unsuitable to form an anode. Moreover, also the combination of such a material and the fluorocarbon polymer layer leads to unreliable performance and therefore to useless devices.
From the above follows that there is still a need in the art for improved structures of electrodes in electronic and opto-electronic devices which show long term stability and high efficiency.
It is therefore an object of the present invention to provide an improved electrode structure for electronic devices comprising an organic material and displays based thereon.